The invention relates to facilities for purifying waste water having biodegradable organic contaminants, and more particularly to arrangements of this nature which employ a separating zone in communication with an aerated activation zone, which moves the waste water through an interface between such zones and into the separating zone, whereby the separated contaminants (in the form of sludge) drop by gravity backward into the aerated activation zone where they are reprocessed by extensive mixing. A disadvantage of such arrangement is that at peak load conditions, the reprocessing of the waste through the essentially single-channel activation zone does not operate efficiently, and an excess of sludge correspondingly appears at the purified water outlet at the top of the separation zone.
In addition, the reaction vessels in which the various zones are defined conventionally take the form of open or closed rectangular vessels, which are difficult and expensive to manufacture and to adjust for variations in load conditions.
In addition, the flexibility of the known arrangements for the treatment of different types of contaminants, such as nitrogenous wastes, is limited since the single activation zone operates at a constant level of aeration which, when adjusted to be satisfactory for the biodegradation of organic contaminants, does not satisfactorily serve to convert the nitrogenous portion of the wastes into gaseous nitrogen.